Use more descriptive synthesis errors now available in bellman.

src/circuit/boolean.rs

@@ -396,7 +396,7 @@ impl<Var: Copy> Boolean<Var> {
                 Ok(())
             },
             Boolean::Constant(true) => {
-                Err(SynthesisError::AssignmentMissing)
+                Err(SynthesisError::Unsatisfiable)
             },
             Boolean::Is(ref res) => {
                 cs.enforce(

src/circuit/mont.rs

@@ -315,7 +315,7 @@ impl<E: JubjubEngine, Var: Copy> EdwardsPoint<E, Var> {
                     Ok(t0)
                 },
                 None => {
-                    Err(SynthesisError::AssignmentMissing)
+                    Err(SynthesisError::DivisionByZero)
                 }
             }
         })?;
@@ -345,7 +345,7 @@ impl<E: JubjubEngine, Var: Copy> EdwardsPoint<E, Var> {
                     Ok(t0)
                 },
                 None => {
-                    Err(SynthesisError::AssignmentMissing)
+                    Err(SynthesisError::DivisionByZero)
                 }
             }
         })?;
@@ -394,7 +394,7 @@ impl<E: JubjubEngine, Var: Copy> MontgomeryPoint<E, Var> {
                     Ok(t0)
                 },
                 None => {
-                    Err(SynthesisError::AssignmentMissing)
+                    Err(SynthesisError::DivisionByZero)
                 }
             }
         })?;
@@ -420,7 +420,7 @@ impl<E: JubjubEngine, Var: Copy> MontgomeryPoint<E, Var> {
                     Ok(t0)
                 },
                 None => {
-                    Err(SynthesisError::AssignmentMissing)
+                    Err(SynthesisError::DivisionByZero)
                 }
             }
         })?;
@@ -480,7 +480,7 @@ impl<E: JubjubEngine, Var: Copy> MontgomeryPoint<E, Var> {
                     Ok(n)
                 },
                 None => {
-                    Err(SynthesisError::AssignmentMissing)
+                    Err(SynthesisError::DivisionByZero)
                 }
             }
         })?;
@@ -579,7 +579,7 @@ impl<E: JubjubEngine, Var: Copy> MontgomeryPoint<E, Var> {
                     Ok(t0)
                 },
                 None => {
-                    Err(SynthesisError::AssignmentMissing)
+                    Err(SynthesisError::DivisionByZero)
                 }
             }
         })?;

src/circuit/num.rs

@@ -272,7 +272,7 @@ impl<E: Engine, Var: Copy> AllocatedNum<E, Var> {
             let tmp = *self.value.get()?;
             
             if tmp.is_zero() {
-                Err(SynthesisError::AssignmentMissing)
+                Err(SynthesisError::DivisionByZero)
             } else {
                 Ok(tmp.inverse().unwrap())
             }